A work machine, such as a construction work machine, an agricultural work machine or a forestry work machine, typically includes a prime mover in the form of an internal combustion (IC) engine. The IC engine may either be in the form of a compression ignition engine (i.e., diesel engine) or a spark ignition engine (i.e., gasoline engine). For most heavy work machines, the prime mover is in the form of a diesel engine having better lugging, pull-down and torque characteristics for associated work operations.
IC engines are used to power work machines under a wide variety of load conditions and must be able to accept sudden changes in load. When the vehicle is in a transport mode, sudden increases in power and torque are required from the engine when negotiating the terrain between fields. Tillage in field also presents conditions where there are sudden increases in load due to changes in soil condition, where the resistance of the tillage tool increases significantly or the field has steep inclines. Engines of this type are expected to respond to these conditions by increasing output torque with only a small increase in engine load. This increase in torque output is typically referred to as torque rise. Engines with significant torque rise permit the torque curve to be shaped so that the rate of rise is very steep allowing the engine to decrease rpm very little at the same time output torque increases significantly. Engines that are governed use the shape of the governor curve to make the slope extremely steep for operation at or below rated rpm and torque. During conditions of higher torque, the shape of the torque limit curve determines the rate of torque rise versus decreasing engine rpm. Significant efforts are applied to shaping the torque limit curve for full throttle operation with the object of giving the tractor its feel of power and responsiveness. Some engine control systems make the curve significantly steep in the first 100-400 rpm in loaded speed below rated rpm. However, this comes at the expense of torque rise at lower engine rpm down to the peak torque of the engine. Steep torque rises encourage the operator to run in this range or at higher speeds because of the sensation of power. In a typical work vehicle equipped with a heavy duty diesel engine, the overall sound and quality of sound both increase as the engine rpm is pulled down by the increasing load requirement. This reinforces the sensation of power and responsiveness. In addition, the change in the rate of torque rise that occurs at approximately rated rpm gives a signal to the operator that the engine is approaching its limit for the operating rpm chosen.
An operational problem occurs when the engine is operated at lower power settings and lower engine rpm. In current systems the governor reduces the torque rise as the engine approaches rated engine rpm. The change in engine response signals the operator that the engine is nearing its torque limit for the chosen operating rpm. At lower power settings, when the engine encounters a step increase in load, the torque rises abruptly. Once the torque reaches the torque limit curve for that particular governor rpm setting, a further increase in torque causes the engine to operate along the torque limit curve in a region where the rate of torque rise per decrease in engine rpm is very low. This occurs because operation under these conditions does not permit the benefit of the shaped portion of the torque curve which provides a warning that the system is moving from the governor range of control to the torque limit. In terms of operator sensation, it appears that a vehicle suddenly runs out of power, (i.e. runs out of its ability to respond to an increase in load).
Work machines such as combine harvesters currently have a basic engine torque curve to provide a nominal rated power at a power level approximately 14% below the power capability envelope of the engine. Experience has shown that a 14% power bulge (from 2,200 rpm rated speed down to 2,000 rpm peak power) provides good slug handling capability and enhanced drivability for the operator. This enables the use of a power boost for unloading or a power bulge for additional power to handle gradual increases in a load or to handle slugs or other operational overloads without excessive loss of functional engine speed or the stalling of the engine. Traditional engine torque curves for combines have been developed to use this high level of power bulge above the normal rated power in order to enhance the ability of the power train and threshing system to handle the slugs and transient overloads during the harvesting operation. Such an overload may occur when clumps of moist material suddenly enter the threshing system causing higher, short duration overloads.
At the lower power end of the operational spectrum, work machines such as combines also spend significant time at very light loads, such as idling or going down hills. In these cases, the high end torque curves that work well for performance, such as slug acceptance, high threshing loads, unloading grain on the go, etc., do not return as good of fuel economy as an engine torque curve optimized for a lower power level operation.
What is needed in the art is an agricultural harvester with an IC engine which operates on a torque curve with improved performance and efficiency.